Electrophoretic display devices are one example of bistable display technology, which use the movement of charged particles within an electric field to provide a selective light scattering or absorption function.
In one example, white particles are suspended in an absorptive liquid, and the electric field can be used to bring the particles to the surface of the device. In this position, they may perform a light scattering function, so that the display appears white. Movement away from the top surface enables the color of the liquid to be seen, for example black. In another example, there may be two types of particle, for example black negatively charged particles and white positively charged particles, suspended in a transparent fluid. There are a number of different possible configurations.
It has been recognized that electrophoretic display devices can enable low power consumption as a result of their bistability (an image is retained with no voltage applied), and they can enable thin and bright display devices to be formed as there is no need for a backlight or a polariser. They may also be made from plastic materials, and there is also the possibility of low cost reel-to-reel processing in the manufacture of such displays.
If costs are to be kept as low as possible, passive addressing schemes are employed. The most simple configuration of a display device is a segmented reflective display, and there are a number of applications where this type of display is sufficient. A segmented reflective electrophoretic display has low power consumption, good brightness and is also bistable in operation, and therefore able to display information even when the power source is turned off.
A known electrophoretic display using a passive matrix and using particles having a threshold comprises a lower electrode layer, a display medium layer accommodating particles having a threshold suspended in a transparent or colored fluid, and an upper electrode layer. Biasing voltages are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrodes being biased.
An alternative type of electrophoretic display device uses so-called “in-plane switching”. This type of device uses movement of the particles selectively laterally in the display material layer. When the particles are moved towards lateral electrodes, an opening appears between the particles, through which an underlying surface can be seen. When the particles are randomly dispersed, they block the passage of light to the underlying surface and the particle color is seen. The particles may be colored and the underlying surface black or white, or else the particles can be black or white, and the underlying surface colored.
An advantage of in-plane switching is that the device can be adapted for transmissive operation, or transflective operation. In particular, the movement of the particles creates a passageway for light, so that both reflective and transmissive operation can be implemented through the material. This enables illumination using a backlight rather than reflective operation. The in-plane electrodes may all be provided on one substrate, or else both substrates may be provided with electrodes.
Active matrix addressing schemes are also used for electrophoretic displays, and these are generally required when a faster image update is desired for bright full color displays with high resolution greyscale. Such devices are being developed for signage and billboard display applications, and as (pixelated) light sources in electronic window and ambient lighting applications. Colors can be implemented using color filters or by a subtractive color principle, and the display pixels then function simply as greyscale devices. The description below refers to greyscales and grey levels, but it will be understood that this does not in any way suggest only monochrome display operation.
The invention applies to both of these technologies, but is of particular interest for passive matrix display technologies, and is of particular interest for in-plane switching passive matrix electrophoretic displays.
Electrophoretic displays are typically driven by complex driving signals. For a pixel to be switched from one grey level to another, often it is first switched to white or black as a reset phase and then to the final grey level. Grey level to grey level transitions and black/white to grey level transitions are slower and more complicated than black to white, white to black, grey to white or grey to black transitions.
Typical driving signals for electrophoretic displays are complex and can consist of different subsignals, for example “shaking” pulses aimed at speeding up the transition, improving the image quality, etc.
Further discussion of known drive schemes can be found in WO 2005/071651 and WO 2004/066253.
One significant problem with electrophoretic displays, and particularly passive matrix versions, is the time taken to address the display with an image. This addressing time results from the fact that the pixel output is dependent on the physical position of particles within the pixel cells, and the movement of the particles requires a finite amount of time. The addressing speed can be increased by various measures, for example providing pixel-by-pixel writing of image data which only requires movement of pixels over a short distance, followed by a parallel particle spreading stage which spreads the particles across the pixel area for the whole display.
Typical pixel addressing times range between several tens to hundreds of milliseconds for small-sized pixels in out-of-plane switching electrophoretic displays up to several minutes for larger-sized pixels in in-plane switching electrophoretic displays. Furthermore, the displacement speed of the particles scales with the applied field. Thus in principle, the higher the applied field, the faster a greyscale change can be achieved, and thus the shorter the image up-date time could be.
However, unfortunately, only at low and very low drive voltages can greyscale uniformity be obtained. Typically, irreproducible and non-uniform greyscales are obtained at the larger drive fields (˜0.1-1 V/μm), or only a low number of shades of greyscales is obtained.
For example, at present the number of accurate (and reproducible) greyscales that can be achieved in commercially available products is just 4. This is unacceptable for e-books and e-signage, which are typically considered to require 4-6 bit greyscales. In general, the greyscale capability in electrophoretic displays depends on a number of critical parameters such as device history, pigment type and pigment non-uniformity, pixel size and pixel-to-pixel non-uniformity, cell-gap and cell-gap non-uniformity, pixel contaminants, temperature effects, pixel design, such as electrode layout, topography, geometry and device operation (drive schemes, addressing cycles/sequences, DC-balancing).